I think this would depend on who strikes first. They're both dawn near unkillable, though I think the Quincy has the most hax of the two. Even with the Almighty banned, I can't see how the Shinigami gets past the Quincy's Antithesis and the Balance passive defences. Never mind X-Axis, the Miracle and the visionary

That said, Mayuri, Urahara and Aizen's combined intellect plus the ability of every shinigami could be prove very deadly.